An impedance tube is a test apparatus that utilizes a speaker to produce a standing wave and microphones to measure the sound characteristics of a sample. Typical sound characteristics measured in an impedance tube are impedance, transmission, and absorption. Currently, a typical impedance tube sample holder is a tube with a constant inner diameter throughout the tube, with the typical sample being foam, or the like. Values can only be reliable if the test sample is oriented perpendicular to the incoming sound wave and if the relative distance of sample to microphones is known. To test acoustical materials, particularly acoustic metamaterials, the sample must be adhered to the tube. Typical impedance tubes require their sample outer diameter be about 0.4% smaller than sample holder diameter. Utilizing rigid samples, the samples are either too large or too small, and not within the required tolerances. Additionally, ASTM standards require certain placement of the sample within the impedance tube. Furthermore, the samples for acoustic metamaterials are prone to rotating within the currently used tube, causing the metamaterial sample to not be oriented perpendicularly to the sound wave. As a result, currently available impedance tubes cannot be utilized to test acoustic metamaterials accurately.